1. Field of Use for the Invention
This invention relates to the field of charged particle optics, and in particular to systems for generation of high current density shaped electron beams.
2. Description of the Related Art
The use of electron beams to lithographically pattern semiconductor masks, reticles and wafers is an established technique. The different lithography strategies may be characterized by the following key parameters: beam positioning strategy; and beam shape control.
There are two main approaches to the positioning of electron beams for the exposure of resist during the lithographic process:                (a) Raster Scanning, where the beam is moved on a regular two-dimensional lattice pattern. This method has the advantage that the scan electronics is typically simpler, but the disadvantage is that the beam may spend large amounts of time moving across areas not needing to be exposed. In addition, in order to accomplish very precise pattern edge placement, sophisticated gray-scale and/or multiple-pass scanning may be required.        (b) Vector Scanning where the beam is moved two-dimensionally directly to areas to be written. This method has the advantage of reduced time over areas not needing to be exposed, but the disadvantage of more complicated and expensive deflection electronics. Precise pattern edge placement is also easier, utilizing the beam placement capability on a 2D address grid much smaller than the beam size.Each approach is advantageous in certain circumstances, the optimum choice depending on the critical dimensions of the pattern, pattern density (% of area to be written), and also on the profile of the beam current distribution.        
There are two well-known approaches to the shaping of the electron beam used to expose the resist on the substrate:                (a) Gaussian beams are characterized by the highest current densities (typically >2000 A/cm2) since in these systems, an image of the electron source is focused onto the substrate surface, thereby taking full advantage of the high brightness of the source. A key disadvantage of Gaussian beams is their long tails of current, stretching far outside the central beam diameter—only 50% of the beam current at the substrate falls within the FWHM of a two-dimensional Gaussian distribution.        (b) Shaped Beams are formed by electron optical columns typically having several intermediate shaping apertures, combined with additional deflectors and lenses to form a focused image of the aperture(s) on the substrate surface. These systems typically have beam current densities orders-of-magnitude lower (e.g. 20-50 A/cm2) than for the Gaussian beams. An advantage of these systems is the reduced current tails outside the desired beam shape, making patterning less susceptible to process fluctuations. Another advantage is that effectively a large number of pixels may be written simultaneously since the area of the variable shaped beam may be large in comparison to a single pixel—this has the effect of increasing the writing throughput since fewer “flashes” of the electron beam are required to write a pattern.        
There is a need in the semiconductor industry to achieve the highest patterning throughputs, both for mask and reticle writing as well as potentially for the direct writing of wafers. Either of the two approaches to beam positioning can be combined with either of the two approaches to beam shaping, but none of these four combinations is capable of fully meeting the semiconductor industry's needs. Clearly there is a need for an electron lithography system having high throughput (at least several wafers/hour or less than an hour to write a reticle), combined with the ability to pattern very small CDs with edge placement accuracies <CD/8, as well as the simplest possible electron optical design to ensure adequate system reliability, long mean-time-between-failures (MTBF) and short mean-time-to-repair (MTTR).
A third possible contribution to increasing throughput is to use multiple beams in parallel to lithographically pattern a single wafer. The challenges associated with using multiple beams include: scaling electron beam columns to fit multiple columns over a single wafer; stitching together the areas patterned by different columns; and the complexity and hardware costs associated with multiple columns.
In order to achieve high throughput, there is clearly a need to have a writing system with two or three of the following characteristics:
1) multiple beams writing in parallel on the same substrate;
2) a high beam current density in a shaped beam;
3) an efficient writing strategy such as vector scanning.
There is a need for a lithography system which makes best use of the above three characteristics.